On Sunday night in Indianapolis, it finally bit the Patriots. The Patriots and Colts each scored touchdowns from inside the 20 three times. The difference is that Indianapolis was only in the red zone three times. The Patriots were there on six occasions.

Forget fourth and 2 for a moment. And realize that in a game that was decided by a single point, New England’s lack of efficiency in the area it counts most provided the opening for the Colts to storm back for their 35-34 win.

“That’s something that we’ve spent extra time working on in the last few weeks, including over the bye week, and we’ll keep working on it,’’ coach Bill Belichick said yesterday. “We’re not doing as well in that area of the field as we’d like to do, as we feel like we can do, and we need to do a better job of it.

“There’s no other way to put it, and those are important points - the difference between 3 and 7 - those are important points both ways and we have to coach it better, we have to play it better. We’ve got to play our best football in that area of the field because there’s a lot at stake and there are a lot of things we need to do better.’’

Among elite offenses, the second-ranked Patriots aren’t alone in red-zone difficulty. The fourth-ranked Cowboys offense is 22d, and the fifth-ranked Giants offense is 27th. On the flip side, the top-ranked Saints attack is sixth in the red zone, and the third-ranked Colts are eighth.

The Colts and Saints are 9-0. The Patriots and Cowboys are 6-3, and the Giants are 5-4.

Yes, it’s that important, and it’s not easy as flipping a switch.

“There’s absolutely a mind-set to it,’’ said ex-NFL quarterback Rich Gannon. “This is who we are, this is what we’re gonna do. It’s a different approach than in the rest of the field. In Oakland, we got inside the 4, and we’d bring [Zach] Crockett in. If you’re Indy, you let Peyton [Manning] throw it.

“But generally, the really good ones bring in heavy personnel, and move people off the ball. Teams like Pittsburgh and Denver, over the years, have run it down there.’’

And that’s maybe where the Patriots’ problem has come. With the arrival of Wes Welker and Randy Moss, the offense was re-formed as a spread attack.

In 2007, the Patriots became the first team in league history to run the majority of their plays out of the shotgun. In 2008, they did it again.

On Sunday night against the Colts, the Patriots had three receivers on the field for 46 of their 72 offensive snaps. They handed to a running back with the quarterback under center on only 13 occasions, and gained just 2.4 yards per carry in those spots.

For the season, only nine of the team’s 28 offensive touchdowns have come on the ground. So the question then would be whether the team’s offensive approach now is creating prolific numbers between the 20s but costing points where it counts most.

“If you don’t run the ball in the field, it’s going to be real hard to do it down there,’’ ex-NFL coach Mike Martz said. “There’s an obvious correlation. I remember with the Rams, when we had a lot of injuries, we started to throw it more than we wanted to down there. We spent an inordinate amount of time on running in the red zone to fix that.’’

Martz’s contention is that having the right backs is critical in an area where the holes close quickly, and decisions need to come fast. And that doesn’t mean the tailback has to be Jerome Bettis, either.

In fact, Martz said that Marshall Faulk was as good a player as he’s seen in that area.

“Our personnel guys were saying we needed a bigger back for goal line,’’ Martz said. “You’d think at 210 pounds, he couldn’t get the ball in the end zone. But after three years with him, you could look back and see how off the charts he was. Some guys, it’s not just plowing in. It’s finding that tiny crease, and bursting into it.’’

Two other positions Martz found vital: a lead fullback to seal off a defender coming unblocked and a big, blocking tight end to seal off the defensive end. Those are two things that, as of right now, the Patriots don’t have as they did in the past. To compensate, New England has used tackle Mark LeVoir as a tight end, and guard/center Dan Connolly as a fullback.

Of course, not every team that excels in the red area plays the big bully. Indianapolis, for example, has a similar breakdown in touchdowns as New England; the Colts have scored 21 through the air, nine on the ground.

“Scheme’s a part of it, but personnel is, too,’’ Gannon said. “You want to have the back to smack it in there. Do you have a tight end like Tony Gonzalez who can cause that matchup problem and jump and make plays down the middle? Do you have a taller receiver who can go up and get the fade?’’

If personnel is the issue, then getting Sammy Morris and Fred Taylor back should help solve the problem, as would, theoretically, the return of a veteran linchpin on the line like Matt Light.

Then, it simply becomes a matter of execution. Martz said he “never carried more than five run [plays] or three pass [plays]’’ into a game to run deep in enemy territory. The idea was to run those plays at the highest level possible in places where the margin for error was thinnest.

Defenses, Martz contended, vary so much team-to-team in the way they play inside the 20, with less ground to cover, that an offense has to do what it does very well to avoid mistakes.

The Patriots, he continued, were once a trend-setter in that area.

“They ran the stretch play there, they were unbelievably successful, and the rest of the league tried to copy them,’’ Martz said. “The whole idea [in the red zone] is to go downhill, and to run sideways and have your back look for that seam went against conventional wisdom. But they did such a great job of teaching it, it worked every time.’’

Things may be simpler down there - mano-a-mano - but yards are tough to come by. Most teams spend significant time on Fridays working on it, and even some of the best offenses scuffle to master it.

For now, the Patriots are Exhibit A of that.

“It’s not just one thing,’’ said Belichick, “but certainly, as a composite on the whole, we need to do it at a higher level.’’